418

ELECTRICAL APPARATUS

t'0 = — = component in phase with e, giving quadrature

flux:

e,

2"0 = — — component in quadrature with e, giving flux in
Q
phase with e.
228. In direct-current motors, overcompensation greatly
improves commutation, and so is used in the form of a com-
pensating winding, commutating pole or interpole. In such
direct-current motors, the reverse field of the interpole produces
a current in the short-circuited armature coil, by its rotation, in
the same direction as the armature current in the coil after
leaving the brushes, and by proper proportioning of the com-
mutating field, the commutation current, ia, thus can be made
to vanish, that is, perfect commutation produced.
In alternating-current motors, to make the commutation
current vanish and so produce perfect commutation, the current
in the short-circuited coil must not only be equal to the arma-
ture current in intensity, but also in phase, that is, the commu-
tating field must not only have the proper intensity, but also
the proper phase.
In paragraph 223 we have seen that the commutating field
has the proper phase to make ig vanish, if produced by a voltage
impressed upon the compensating winding:
#2 = Te,
which for all except very low speeds is very nearly in phase with
e. The magnetic flux produced by this voltage, or the com-
mutating flux, so is nearly in quadrature with e, and therefore
approximately in quadrature with the current in the motor,
at such speeds where the current, i, is nearly in phase with e.
The commutating flux produced by conductive overcompensa-
tion, however, is in phase with the current, i, hence is of a
wrong phase properly to commutate.
That is, in the alternating-current commutator motor, the
commutating flux should be approximately in quadrature with
the main flux or main current, and so can not be produced by
the main current by overcompensation, but is produced by the
combined magnetizing action of the main current and a sec-
ondary current produced thereby, since in a transformer the re-
sultant flux lags approximately 90° behind the primary current,